how to check if content txt file has changed

Situation: I am reading the content of a .txt file with php and with AJAX i load the content into a div. The javascript checks every 5 seconds the .txt file and put the content into the div. If the content of the .txt file changes, (which i do with a form submit), the content of the div changes automatically after 5 seconds. For this; i use a checkbox with 3 options: Status: Available Status: Busy Status: Paused

One of the 3 lines above is in the .txt file. Situation now: every 5 seconds check of the .txt file and every 5 seconds refresh of the div. Is it possible that if the content of the .txt file has not changed, to keep the refresh away?

How can i achieve this?

Below the javascript:

 function Ajax()
    {
        var
            $http,
            $self = arguments.callee;

        if (window.XMLHttpRequest) {
            $http = new XMLHttpRequest();
        } else if (window.ActiveXObject) {
            try {
                $http = new ActiveXObject('Msxml2.XMLHTTP');
            } catch(e) {
                $http = new ActiveXObject('Microsoft.XMLHTTP');
            }
        }

        if ($http) {
            $http.onreadystatechange = function()
            {
                if (/4|^complete$/.test($http.readyState)) {
                    document.getElementById('ReloadThis').innerHTML = $http.responseText;
                    setTimeout(function(){$self();}, 5000);
                }
            };
            $http.open('GET', 'loadtxt.php' + '?' + new Date().getTime(), true);
            $http.send(null);
        }

    }

Loadtxt.php

<?php
        //
        $file = "status.txt";
        $f = fopen($file, "r");
        while ( $line = fgets($f, 5000) ) {
        echo $line;
        }
?>

The div:

<script type="text/javascript">
    setTimeout(function() {Ajax();}, 5000);
</script>
<div id="ReloadThis">Default text</div>